1. Field of the Invention
The present invention relates to a navigation apparatus and navigation processing method, and is preferably applied to a navigation apparatus mounted on a vehicle, for example.
2. Description of Related Art
Generally, a navigation apparatus to be mounted on a vehicle and the like is here to stay. The navigation apparatus calculates the current position of the vehicle based on a GPS signal from a Global Positioning System (GPS) satellite. The navigation apparatus displays on a display section a map with information such as the current location and speed.
In addition, there is a portable navigation apparatus that can be taken out of the vehicle to use it in a different vehicle. This also prevents theft while the vehicle is parked. This type of navigation apparatus is not directly connected to the electronic circuits of the vehicle even when it is inside the vehicle. Accordingly, while unable to receive the GPS signals due to the vehicle running inside a tunnel or behind building, the navigation apparatus may not be able to calculate the vehicle's speed from a vehicle's speed pulse supplied from the vehicle.
On the other hand, there is a navigation apparatus that calculates the vehicle's current location, travel distance and the like even when not receiving the GPS signals: The navigation apparatus autonomously estimates, based on the vehicle's speed calculated from the GPS signals (This calculated speed is also referred to as “GPS velocity”), the vehicle's speed and then calculates the vehicle's current location, travel distance and the like based on the estimated speed (This estimated speed is also referred to as “autonomic speed”). In this manner, the navigation apparatus autonomously estimates the vehicle's speed even when not receiving information from the outside.
However, the GPS velocity calculated by the navigation apparatus is not always accurate, including a certain degree of error when the GPS signals are not received appropriately. On the other hand, when receiving the GPS signals, the navigation apparatus calculates the current position of the vehicle from the GPS signals, not from the GPS velocity. In this case, the calculated current position is substantially accurate because it is not affected by errors of the GPS velocity.
However, when not receiving the GPS signals, the navigation apparatus autonomously estimates the vehicle's speed as the autonomic speed, based on the GPS velocity. In addition, the navigation apparatus calculates, based on the estimated autonomic speed, the vehicle's travel distance or a distance the vehicle has traveled so far while not receiving the GPS signals. The navigation apparatus subsequently estimates the vehicle's current position based on the calculated travel distance. In this case, the estimation of the current position may be affected by the errors of the GPS velocity.
That is, the autonomic speed estimated by the navigation apparatus may be less accurate if the GPS velocity includes errors. The vehicle's travel distance calculated from the autonomic speed may become less accurate accordingly. The travel distance becomes less accurate as time advances because it is affected by the errors of the autonomic speed. Accordingly, the estimation of the vehicle's current position becomes less accurate as time advances. In this manner, the estimated current position (also referred to as “autonomic position”) becomes less accurate as time advances.
By the way, as disclosed in Patent Document 1 (Jpn. Pat. Laid-open Publication No. H10-38592), there is a navigation apparatus that stops displaying the autonomic position after a predetermined period of time has passed since the start of the autonomic speed calculation.